Why they try to diminish the greatness of Prophet Muhammad

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Each of these scholars has a 'dal', d. before his name. Why does he need to put 'dal' before his name? To tell everyone that he is a "doctor". Yet, he doesn't accept that the Prophet Muhammad, upon whom be peace and blessings of Allah, the one who went to "two bow's-length or nearer" to the Divine Presence, is not able to prescribe medicine to humankind. What is the benefit of their "doctorship" then!?